Update event_mon proto

R=tandrii@chromium.org
BUG=579589, 596226

Review-Url: https://codereview.chromium.org/2130713002
Cr-Mirrored-From: https://chromium.googlesource.com/infra/infra
Cr-Mirrored-Commit: e2f882a995870b9803fbb794ff662ae11211ce24
1 file changed